Video signal processing

ABSTRACT

In motion compensated standards conversion, an output video field is constructed from four input fields (two inner fields and two outer fields) by shifting pixel information from an outer field to the output field position in two stages, the first stage using a first motion vector between the outer field and the adjacent inner field and the second stage using a second motion vector being derived from a motion vector pointing from one inner field to the other inner field.

[0001] This invention relates to video processing and particularly tomotion compensation techniques.

[0002] The use of motion compensation is well known in a variety ofprocesses, of which standards conversion is a good example. In one priorart approach, each output field is constructed from a number of storedinput fields. For a given output pixel, information is read from thepixels in the input fields in conjunction with the motion vectorassociated with the required pixel. A problem associated with thisapproach is that the motion vectors must first have been converted tothe new standard, causing extra complexity and creating artefacts.

[0003] A different approach to motion compensated standards conversionis detailed in EP 0 648 398. Here, the motion vectors are employed inwriting the input fields to a store and the output picture isconstructed as data is accumulated in the store. This method of approachhas a number of advantages, including use of motion vectors at the inputstandard. There can sometimes be a difficulty, however, withdiscontinuous motion.

[0004] When there is discontinuous motion in the video source to beconverted, the motion vectors from the outer of the four input fieldsmay, when extrapolated to the output field position, point to differentpositions for the output pixel, creating aliasing and multiple images.

[0005] It is hence an object of the present invention to provide amotion compensated standards conversion system that results in a sharpoutput even in the face of discontinuous motion.

[0006] Accordingly, the invention consists in one aspect in a method forconstructing an output video field from at least four input fieldscomprising two inner fields and two outer fields, wherein pixelinformation from an outer input field is temporally shifted to theoutput field position in two stages, the first stage using a firstmotion vector between the outer field and the adjacent inner field andthe second stage using a second motion vector being derived from amotion vector pointing from said inner field to the other inner field.

[0007] A preferred embodiment of the invention will now be described byway of example, with reference to the accompanying drawings, in which:

[0008]FIG. 1 is a set of diagrams illustrating the operation of thepresent invention, by way of comparison with prior art; and

[0009]FIG. 2 is a block diagram of illustrating an embodiment of thepresent invention.

[0010] Referring initially to FIG. 1, diagrams A and B show a prior artapproach to the construction of an output field from four input fields.Each of the input fields is individually shifted by a motion vector tothe temporal position of the required output field. In each case themotion vector is derived from a motion measurement conducted between thefield to be shifted and an adjacent field; the measurements for theouter fields being “scaled up” to the required position, and themeasurements for the inner fields being “scaled down” to the requiredposition.

[0011] Diagram A shows that with continuous motion, the prior art isable to produce sharp images. It is clear from diagram B, however, thatpresented with non-continuous motion, the prior art approach may resultin multiple images.

[0012] Diagrams C and D illustrate the present invention. It will beseen that pixel information from the outer input fields is brought tothe output field position, not by extrapolation of vectors but in atwo-stage process. First, the outer fields are shifted to the respectivepositions of the adjacent inner fields by respective vectors measuredbetween the outer and inner fields. Then the results, together with theinner input fields, are shifted to the required output field position byrespective interpolated vectors derived from motion measurements betweenthe two inner fields. Thus in the first stage, the shift is the resultof a measurement between consecutive fields and in the second, it is theresult of scaling down a measurement between consecutive fields. Becausethere is no extrapolation of vectors, a sharp image always results.

[0013] This method also therefore holds significant advantages overknown methods such as “vector tracing”, in which a succession of motionmeasurements is made between adjacent fields and the resulting vectorsare summed to obtain a vector describing the motion between noncontiguous frames. In each stage of the embodiment illustrated inDiagrams C and D, information is carried no further than the distancebetween successive input samples. This produces sharper images inmaterial having discontinous motion, as misleading motion informationfrom “old” frames is not reproduced in the output field.

[0014] A further advantage is that there is less variation in verticalresolution as the temporal phase of the output field (with respect tothe input fields) changes.

[0015] It should be noted that the invention differs from the knownmethod of. In the invention the motion compensation is carried out instages and

[0016] A motion compensated standards converter according to anembodiment of the invention will now be described. Referring to FIG. 2,an input video signal (10), which is accompanied by motion vectors froma motion estimator (not shown), is taken to a store (11). The store hasfour outputs (F1, F2, F3, F4) which provide the four fields which“straddle” the required output field. The two earlier fields are inputto a motion compensated de-interlacer (12) and the two later fields areinput into a similar motion compensated de-interlacer (13).

[0017] The de-interlacer (12) constructs a progressive picture at thetemporal position corresponding to F2. Half of its output lines aretaken directly from the input F2 lines, and the intervening lines arecreated using lines from both F1 and F2 together with motion vectors(14) measured between F2 and F1. High vertical frequencies from F1 arederived by the filter (15) and shifted to the temporal positioncorresponding to F2 by the pixel shifter (16). Low vertical frequenciesare separated from F2 in the filter (17) and added to the temporallyshifted high frequencies in the adder (18).

[0018] The second de-interlacer (13) operates in a similar way to createa progressive picture at the temporal position corresponding to F3 byusing motion vectors (19) measured between F3 and F4.

[0019] It should be noted that it is advantageous for the filter (15) toprecede the shifter (16). This is because any motion estimation errorsin plain areas of the picture will have no effect. If the shifter wereto precede the filter, false edges due to motion measurement errorswould pass through the high-pass filter and create artefacts.

[0020] The outputs from the de-interlacers (12) and (13) are connectedto pixel shifters (19) and (20). These move the two outputs to thetemporal position of the required output field. The shifters arecontrolled by respective motion vectors (21) and (22) which are derivedby appropriate (down) scaling of the measured motion between F3 and F2,and between F2 and F3.

[0021] The two resulting (co-timed) output fields are combined in thecombiner (23). This may be a simple averager or may vary thecontributions in dependence upon various parameters which will beapparent to those skilled in the art. The output of the combiner is fedto a line standards converter (24) which carries out any necessaryvertical spatial interpolation to arrive at the required number ofoutput lines and, if necessary, to introduce interlace.

[0022] It should be understood that this invention has been described byway of examples only and that numerous modifications are possiblewithout departing from the scope of the invention. For example: thede-interlacers (12) and (13) may use more than two fields; the pixelshifters (16), (19) and. (20) may be based on “write-side” architecturein which the write addresses of a store are modified in dependence uponthe motion vectors, or based upon “read-side” architectures in whichread addresses are manipulated in dependence on motion vectors; theshifters may, or may not, be capable of sub-pixel or sub-line shifts;and, the combination of fields may be modified when cuts or shot-changesare detected.

1. A method for constructing an output video field from at least fourinput fields comprising two inner fields and two outer fields, whereinpixel information from an outer input field is temporally shifted to theoutput field position in two stages, the first stage using a firstmotion vector between the outer field and the adjacent inner field andthe second stage using a second motion vector being derived from amotion vector pointing from said inner field to the other inner field.2. A method according to claim 1, wherein for the outer field closest tothe output field position, the first motion vector is that pointingbackward from the adjacent inner field.
 3. A method according to claims1 or claim 2, wherein only high frequency information is taken from theouter input fields.
 4. A method according to claim 3, wherein said highfrequency information is vertical information.
 5. A method according toclaim 3 or claim 4, wherein the high frequency information is taken fromthe outer input fields before being temporally shifted.
 6. A motioncompensated video process in which information from temporallynon-consecutive input samples is combined, characterised in that wheremotion compensation is performed over a time interval greater than thetime between consecutive samples, the compensation is performedseparately between consecutive temporal samples.
 7. A method accordingto claim 6, wherein the motion compensation is carried out in stages,and wherein in each stage information is carried no further than thetemporal distance between successive input samples.
 8. A motioncompensation device for constructing an output video field from at leastfour input fields comprising two inner fields and two outer fields,comprising a store and means for temporally shifting pixel information,and being adapted to temporally shift pixel information from an outerinput field to the output field position in two stages, the first stageusing a first motion vector between the outer field and the adjacentinner field and the second stage using a second motion vector beingderived from a motion vector pointing from said inner field to the otherinner field.